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Biogeochemical data from an urban tropical estuary implicate unexpected nitrogen sources
Citation:
Oczkowski, A., E. Santos, R. Martin, E. Huertas, Alana Hanson, E. Watson, AND C. Wigand. Biogeochemical data from an urban tropical estuary implicate unexpected nitrogen sources. 2021 Annual Meeting of the Association for Tropical Biology and Conservation, Virtual, COLOMBIA, July 21 - 23, 2021.
Impact/Purpose:
Coastal cities in tropical areas are often low-lying and vulnerable to flooding. Portions of San Juan, Puerto Rico are a good example of this. A canal that used to connect lagoons in the San Juan Bay Estuary has filled in with sediment and debris and the communities adjacent to the canal are frequently flooded with sewage-enriched waters. Here we look at the biogeochemistry of the sediments in the estuary and the peat of the mangrove forests to document the ecological impact of the closed canal on the whole San Juan Bay Estuary. As tropical urban areas are so understudied, we make new observations about nutrient cycling in the region and suggest that the polluted flood waters create conditions that enhance the contributions of bioavailable nitrogen via nitrogen fixation by microbes. Thus, compounding water quality issues in the region.
Description:
Tropical urban estuaries are understudied. These often low-lying city-estuarine ecosystems, and the people who live in them, are particularly vulnerable to hazards associated with hurricanes and storm surges. The San Juan Bay Estuary (SJBE) in San Juan, Puerto Rico is an example of such a system. It has been heavily altered by dredging, filling, and extensive shoreline modification for more than two centuries. Currently, a channel in the SJBE, the Caño Martín Peña, has filled in with debris and sediment. While it was 2-4 m deep and 60 m wide in the 1930s, it can now be crossed on foot in parts. The closure of this channel likely has far reaching effects on ecosystem functioning as it has eliminated the connectivity of San Juan Bay with the adjacent lagoonal portion of the estuary. Here we used historical SJBE data and our own ecological and biogeochemical measurements to assess how changes in the hydrodynamics have impacted nutrient cycling in this tropical urban estuary. Our goals were to identify the areas most impacted by the Caño’s closure and to begin to identify how recent hydrodynamic shifts may be influencing the water quality of the poorly flushed portion of the SJBE that lies east of the Caño. Sediments in and near the Caño had high levels of organic matter (40-50%) and organic nitrogen (N; 1.5-2%). This area is characterized by ponded water and receives high amounts of untreated urban runoff and sewage from the adjacent communities. Despite this, N stable isotopes (δ15N) in the surface sediments were the lowest in the SJBE, with values ranging from 1-4‰. In contrast, sediments from the adjacent lagoon, which has a residence time of >17 days (increased from ~3 days in the 1930s) were the highest in the SJBE (>6‰), consistent with a settling basin where urban runoff and organic matter is broken down and denitrified. The δ15N data suggest that the most urban, sewage-enriched portion of the SJBE is also fixing large amounts of N. While N-fixation is well-documented in mangrove ecosystems, often associated with sulfate reduction, we find it surprising that it may be such a dominant process in urban mangroves.
